Re: [PATCH v2 1/3] perf/core: Add a tracepoint for perf sampling

[Peter Zijlstra]

On Thu, Aug 04, 2016 at 10:24:06PM -0700, Alexei Starovoitov wrote:
> tracepoints are actually zero overhead already via static-key mechanism.
> I don't think Peter's objection for the tracepoint was due to overhead.

Almost 0, they still have some I$ footprint, but yes. My main worry is
that we can feed tracepoints into perf, so having tracepoints in perf is
tricky.

I also don't much like this tracepoint being specific to the hrtimer
bits, I can well imagine people wanting to do the same thing for
hardware based samples or whatnot.

> > The perf:perf_hrtimer probe point is also reading state mid-way
> > through a function, so it's not quite as simple as wrapping the
> > function pointer. I do like that idea, though, but for things like
> > struct file_operations.

So what additional state to you need?

> > > Currently overflow_handler is set at event alloc time. If we start
> > > changing it on the fly with atomic xchg(), afaik things shouldn't
> > > break, since each overflow_handler is run to completion and doesn't
> > > change global state, right?

Yes, or even a simple WRITE_ONCE() to replace it, as long as we make
sure to use a READ_ONCE() to load the pointer.

As long as we're sure to limit this poking to a single user its fairly
simple to get right. The moment there can be concurrency a lot of fail
can happen.

> instead of adding a tracepoint to perf_swevent_hrtimer we can replace
> overflow_handler for that particular event with some form of bpf wrapper.
> (probably new bpf program type). Then not only periodic events
> will be triggering bpf prog, but pmu events as well.

Exactly.

> So instead of normal __perf_event_output() writing into ringbuffer,
> a bpf prog will be called that can optionally write into different
> rb via bpf_perf_event_output. 

It could even chain and call into the original function once its done
and have both outputs.

> The question is what to pass into the
> program to make the most use out of it. 'struct pt_regs' is done deal.
> but perf_sample_data we cannot pass as-is, since it's kernel internal.

Urgh, does it have to be stable API? Can't we simply rely on the kernel
headers to provide the right structure definition?